1. Introduction
This invention relates to immersion tin-lead alloy deposition and more particularly, to replenishment of an immersion tin-lead depositing solution during use of the solution to maximize adhesion between the deposit and an underlying substrate. The invention is especially useful for the manufacture of printed circuit boards.
2. Description of the Prior Art
Tin-lead alloy having a composition of between 60 and 65 percent tin and 35 to 40 percent lead (solder) is coated onto copper traces in the manufacture of printed circuit boards. The alloy may function as an etch resist and permits attachment of components to a circuit board. For this latter use, the alloy must melt and reflow at a reasonably low temperature to avoid damage to the circuit during heating of the board and must have an adequate thickness for surface mounting of components.
Conceptually, there are four methods available for coating tin-lead alloys onto copper traces in printed circuit manufacture. These methods include (1) hot air leveling, (2) electroless plating, (3) immersion plating and (4) electroplating. Each process has advantages and disadvantages and the metal deposit obtained from each differs from the deposit obtained by other plating methods.
Electrolytic plating of tin-lead alloys is a method of choice but suffers certain disadvantages. For example, a circuit board must be racked as part of an electrolytic cell which is labor intensive. Throwing power of the electrolytic bath does not provide desired uniformity with deposits of uneven thickness obtained, especially where high aspect ratio through-holes are encountered. Deposits of a 60/40 percent tin-lead alloy require close control of the solution composition and operating conditions of the electrolytic cell. Finally, damaged circuits are not easily reworked because electrolytic deposition cannot take place over a discontinuous conductive substrate.
Hot air leveling is conventionally used to provide a tin-lead deposit over copper circuits. In this process, the circuit board is floated or immersed in a molten solder bath. The coating obtained is not uniform but is made more uniform by passing the board with the solder coating beneath a hot air stream to level the coating. Even with the hot air stream, the coating formed is thin at the heel of the hole and uneven and thin in the center of the hole when high aspect ratio holes are plated.
Electroless plating baths for each of tin and lead are known but a publication showing electroless codeposition of tin and lead is not known. It is believed that electroless solder plating baths are difficult to formulate because an autocatalytic solution would contain a mixture of tin and lead as well as a complexing agent and reducing agent in a single plating solution.
Immersion plating is an electroless plating process, but is separately classified because deposition is by displacement of elemental metal from a substrate by metal ions in a plating solution. In contrast, in electroless plating, plating takes place primarily by autocatalytic reduction of metal ions from solution.
Since immersion plating is by displacement, immersion plating is an electrochemical reaction dependent upon the position the substrate metal occupies in the electromotive series relative to the metal to be deposited from solution. Plating occurs when the metal from a dissolved metal salt is displaced by a more active (less noble) metal immersed in the solution. When tin and lead are complexed under acidic conditions, the electropotentials of the tin and lead complexes relative to copper make immersion plating possible. However, limitations in the use of immersion plating for circuit fabrication exist. These include a slow plating rate, difficulty in obtaining a desired alloy and limited deposit thickness. Limited thickness is due to the plating reaction being self limiting because as the coating builds, the metal deposited from solution masks the underlying base metal required for displacement. Additionally, as the displaced base metal is dissolved in solution, it becomes a contaminant progressively slowing the rate of displacement. Typical deposit thickness for an immersion tin deposit in the prior art is 50 to 100 microinches, mainly because of the foregoing problems.
Absent the above disadvantages, there are advantages to immersion plating compared to other methods for depositing tin-lead alloy in circuit manufacture. For example, compared to electroplating, there is no hydrogen generation during the plating process thus avoiding pitting and discontinuities in the deposit. Also, immersion plating is not subject to surface roughness as found in electroplating processes due to "drag-over" from precleaners, anode corrosion and the like. In immersion plating, spontaneous plating is not a problem as would likely be encountered with autocatalytic plating. Moreover, with immersion plating, neither an electrically continuous circuit nor attachments of electrical contacts are required nor is there a need to maintain a precise current. Finally, immersion deposits are conformal in nature and conform to the topography of the substrate over which they are plated.
Despite the potential advantages of immersion plating, the prior art dismissed immersion plating processes for printed circuit fabrication because it was believed that thick, bondable (solderable) deposits were unavailable. As stated in Printed and Integrated Circuitry, McGraw Hill Book Company, Inc., New York, 1963 at page 138, immersion deposits are "limited in thickness, porous, and often poorly adherent and, therefore, of limited interest." The self limiting feature of immersion tin and lead plating was believed to make soldering impossible and consequently, plating baths for immersion deposits of tin and lead were of minimal interest to the art.
In U.S. Pat. No. 4,194,913, incorporated herein by reference, an immersion plating composition is disclosed for deposition of tin-lead alloys. In accordance with the teachings of this patent, an immersion plating solution is disclosed comprising stannous chloride, lead chloride, sodium hypophosphite, thiourea, hydrochloric acid and gelatin. In this patent, patentee states that the plating composition provides a faster plating rate and a deposit of increased thickness. Though it is believed that improved immersion deposits are obtained using the compositions of the patent, it is believed that the deposits are unsuitable for the commercial manufacture of printed circuit boards.
In European published application No. 0 167 949, also incorporated herein by reference, an immersion plating solution is disclosed that may contain a tin and lead salt of a fluorine containing mineral acid, a fluorine containing mineral acid sufficient to provide a pH varying up to 1, and a sulfur containing complexing agent such as thiourea. An example of a tin-lead plating solution is given though it is believed that thick, reflowable deposits of tin and lead are not obtained and the compositions are believed to be unsuitable for the circuit fabrication.
In copending U.S. patent application Ser. No. 07/532,819, now abandoned, filed Jun. 4, 1990 assigned to the assignee hereof and incorporated herein by reference (hereafter the "copending application"), an immersion plating solution is disclosed capable of plating a thick tin-lead alloy deposit capable of reflow at a temperature of below about 500.degree. F. The invention of the copending application was based upon a combination of discoveries. One such discovery was that to reflow a deposit, it was necessary that the deposit be porous and have a thickness of at least 100 microinches. Another discovery was the realization that to obtain a thick deposit capable of reflow, it was necessary to use a solution favoring a porous structure rather than a dense deposit. To obtain such a deposit, an immersion plating solution containing a relatively high metal content was used - i.e., preferably in excess of 0.10 moles per liter of total metal with a tin to lead ratio of at least 1 to 1. Further, both the tin salt and the lead salt, and preferably the acid used to provide a desired pH, all had a fluorine containing anion. In addition, recognizing that the displacement reaction favored tin, a lead promoter was added to solution to maintain a desired concentration of lead relative to tin and to obtain a thick deposit. Finally, to obtain as thick a deposit as possible, exaltants were added to the formulation.
Regardless of the immersion plating solution used, an immersion deposit should adhere firmly to the underlying substrate to prevent delamination during use. In practice, it has been found that over the useful life of a tin lead immersion plating solution such as that of the copending application, adhesion of the immersion deposit to its underlying substrate decreased. Therefore, it is desirable to find a means for preventing this decrease in adhesion.